In the case of loading trucks or ships with coated automobiles, the parts thereof, etc., and transporting them to remote places such as overseas, etc., an effective means for preventing the paint films from being damaged, dulled, discolored, etc., by various suspensions or collisions of dusts, rains, etc., has been desired.
Hitherto, a method of coating a coating material comprising a wax as the main component in a thickness of from 5 to 40 .mu.m has been known as such a preventing method. However, this method has various problems that it is difficult to form the wax coating at a uniform thickness, whereby a uniform protection is not obtained; the coated film is liable to be stained and has a poor resistance to acidic rain; the wax, etc., penetrate into the coated paint film to cause a discoloration, etc.; the application of the wax coating and the removal thereof require much labors; the use of a solvent, the treatment of waste liquids, etc., are liable to cause environmental problems, etc.
On the other hand, various surface protective sheets each comprising a supporting substrate having formed thereon a pressure-sensitive adhesive layer are known, and also a surface protective sheet comprising a supporting substrate having formed thereon a radiation-curable pressure-sensitive adhesive layer having a lowered glass transition point (Tg) is proposed for use on an adherend having a paint film as disclosed in JP-A-2-199184 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Such sheet systems can overcome the problems described above.
However, when such a surface protective sheet is adhered to the surface of the paint film of an automobile, etc., low molecular weight components contained in the paint film, such as a fatty acid glycerol ester, etc., migrate into the surface protective sheet side due to the temperature rise in an outdoor transportation, etc., whereby a step mark phenomenon occurs in the paint film to cause a problem that adhesive marks of the protective sheet clearly appear on the surface of the paint film. In particular, in a low Tg type paint film, the adhered traces having a depth of from about 0.1 to 0.5 .mu.m usually occur during the adhesion of the protective sheet for about 10 hours under 50.degree. C.
The present inventors have previously proposed a surface protective sheet in which a rubber-based polymer such as a polyisobutylene is used as a pressure-sensitive adhesive (EP 0 519 278 A2). This sheet is hard to cause the deformation of a paint film and does not have a problem on adhesive remaining. Thus, this sheet is a practical surface protective sheet. However, this sheet has in some cases a problem that the adhesive marks may not completely be eliminated where the paint film contains a fatty acid glycerol ester.